CNTs have excellent electrical characteristics and mechanical strength along with other superior properties, and have been actively researched and developed as an ultimate novel material. CNTs are synthesized by using various methods, including a laser vaporization method, an arc discharge method, and a chemical vapor deposition method (CVD method). However, CNTs produced by using any of the currently available synthesis methods are obtained as a mixture of metallic CNTs and semiconducting CNTs.
Because either one of the metallic and semiconducting properties is often used in actual use, separation and purification of only the metallic or semiconducting CNTs from a CNT mixture is an urgent and important research subject.
There are reports of separating metallic CNTs and semiconducting CNTs. However, all of these reports pose problems in industrial production of metallic CNTs and semiconducting CNTs, as follows. (1) The separation involves complicated steps, and cannot be automated; (2) the separation is time consuming; (3) mass processing is not possible; (4) expensive equipment and chemicals are required; (5) only one of the metallic CNTs and semiconducting CNTs are obtained; and (6) the collection rate is low.
Examples of the currently available methods include a method in which CNTs dispersed with a surfactant are subjected to dielectrophoresis on microelectrodes (Non-Patent Document 1), a method in which amines are used as the dispersant in a solvent (Non-Patent Documents 2 and 3), and a method for selectively burning semiconducting CNTs with hydrogen peroxide (Non-Patent Document 4). However, these techniques also have the foregoing problems. Particularly, the final material is limited to metallic CNTs, and the collection rate is low.
Other methods includes a method in which a mixture of semiconducting CNTs and metallic CNTs is dispersed in a liquid to selectively bind the metallic CNTs to particles, and in which the metallic CNTs attached to the particles are removed to separate the semiconducting CNTs (Patent Document 1), a method in which CNTs treated with a nitronium ion-containing solution are filtered and heat treated to remove the metallic CNTs contained in the CNTs and to obtain semiconducting CNTs (Patent Document 2), a method using sulfuric acid and nitric acid (Patent Document 3), and a method in which CNTs are selectively separated by migration under applied electric field to obtain semiconducting CNTs confined within a narrow electric conductivity range (Patent Document 4).
These techniques also have the foregoing problems. Particularly, the resulting final material is limited to semiconducting CNTs, and the collection rate is low.
There is also a method in which CNTs dispersed with surfactants are separated into metallic CNTs and semiconducting CNTs by density-gradient ultracentrifugal separation (Non-Patent Document 5). This technique is also problematic, because the method uses a very expensive ultracentrifugal separator, and requires a long time for the ultracentrifugal separation procedure. Further, because the ultracentrifugal separator can only be increased to a certain size, more than one ultracentrifugal separator needs to be installed in parallel, and accordingly automation and other processes are difficult.
In another method, separation is achieved by ion-exchange chromatography using a CNT-nucleic acid complex of CNTs attached to nucleic acid molecules (Patent Document 5). A problem of this method, however, is that it requires an expensive synthetic DNA, and that the collection rate and purity are poor because of the moderate separation accuracy.
Further, there is a report directed to separating metallic CNTs and semiconducting CNTs under electric field after causing protonation in different extents for different CNTs by adjusting the pH or ion strength of a CNT solution prepared by dispersing the CNTs with a surfactant (Patent Document 6). However, in this method, a pretreatment step needs to be performed before separation with the use of a strong acid for the pH and ion-strength adjustments of a suspended nanotube mixture. The method thus inevitably involves strict step control, and does not successfully separate metallic CNTs and semiconducting CNTs (Patent Document 6, Example 4).
As described above, all of the conventional methods are insufficient for overcoming the foregoing problems, and there is a need for developing a method based on new ideas whereby metallic CNTs and semiconducting CNTs can be separated from CNTs.
The present inventors have worked on a novel method that differs from any of the conventional methods for separating metallic CNTs and semiconducting CNTs, and completed the inventions below (Patent Documents 7 to 9, and Non-Patent Documents 6 to 8). These inventions all use gels, and desirable separation was confirmed particularly when an agarose gel was used. It was found for the first time that separation of metallic and semiconducting CNTs is possible by agarose gel electrophoresis that uses a solution of CNTs dispersed in a surfactant (Patent Document 7, Non-Patent Document 6). The present inventors also invented a high-yield separation method whereby nearly all CNTs are separated into metallic CNTs and semiconducting CNTs by the electrophoresis of CNTs not in the solution state but in the solidified state in a gel (CNT-containing gel) (Patent Document 8, Non-Patent Document 6). It was also found that the separation using the CNT-containing gel is also possible by applying physical means such as centrifugation, freezing-thawing-squeezing, diffusion, and permeation, instead of using electrical means such as electrophoresis (Patent Document 9, Non-Patent Document 7). This technique allows CNTs to be inexpensively separated in larger quantities and more easily than the techniques that use electrophoresis. In all of the techniques above, separation is achieved by the selective adsorption of the semiconducting CNTs to the gel, and requires dissolving the gel for the collection of the semiconducting CNTs adsorbed to the gel. The present inventors have developed a technique that uses an appropriate eluant for the CNT collection, without dissolving the gel (Non-Patent Document 8). Particularly, the continuous separation method that takes advantage of the adsorption and desorption based on the chromatography technique enables the adsorbed CNTs to be collected in the solution state, and the gel to be directly reused. Further, the technique is highly desirable, because it allows the separation to be automated, and improves the purity of the separated CNTs. However, while the gel can be repeatedly used in this separation method, fast separation requires increasing the surface area with the use of fine gel particles, and using spherical and uniform gel beads to provide space between the gel particles for the passage of the solution. Such fine gel beads of a uniform shape are often expensive, and alternative materials are strongly needed.